OverviewIn modern, general purpose CPUs, the accumulator register has lost some of its meaning and much of its use has been relegated to convention rather than necessity.
I got to wondering why the ax/eax/rax register was called the accumulator and why it was used so much since it seemed that any of the general purpose registers would do. Does it just come down to convention or is there more to the story?
All the MIPS documentation I’ve read to date indicates the only register considered the accumulator is a hidden, double-word register accessed via the MFHI and MFLO instructions. This speical register is used when doing multiplication and division.
In developing my own virtual machine, and delving into electrical engineering, I got to be more intimately familiar with this happy little register.
A Little HistoryIf my sources are correct, the first commercially available CPU from Intel was the 4004. It had a single register that was implictly used in the vast majority of its instructions. You can see for yourself on the data sheet (PDF). Can you guess which one?
The 4004 was designed in such a way that the output of the ALU fed directly into this single register to accumulate data. Operations that expect two operands always implicitly use the accumulator as the first operand.
The ADD instruction, for example, takes a single register as an operand. It performs addition on the value stored in the accumulator plus the given register and then stores the result back into the accumulator.
To my knowledge, the 4004 did not pioneer the use of Accumulator but it serves as a nice example.
ExampleThe design seems simple but consider how calculators or adding machines work. How about an example?
Assume the calculator has just been turned on or a clear instruction has been set so that the accumulator has a value of zero.
The user types in a value then hits the addition key. The accumulator and the value on screen get added together. There’s no operands just a simple ADD instruction. ACC += DISPLAY.
Arithmetic instructions would not need explicit operands at this level of design. It makes the accumulator a very important facet in the design of these machines.
In modern implementations, even if a CPU can use any general purpose register to execute a particular instruction, it may be optimized to perform certain operations in fewer clock cycles when using the accumulator. You’ll need to read documentation to find out.
LegacyThe result is that the importance of this register is historic and thereby remains important to standard convention. We probably no longer need to do it this way but it does have a logical, and historic, significance.